1. Field of the Invention
The present invention relates to liquid diphenylmethane diisocyanate (MDI) prepolymers having urea or biuret groups. More specifically, the invention relates to prepolymers which are prepared by reacting MDI and aminocrotonates.
2. Brief Description of the Prior Art
Urea and biuret prepolymers are generally known in the art. U.S. Pat. No. 3,862,973 discloses biuret-containing polyisocyanates which are prepared by reacting organic polyisocyanates with secondary amines at temperatures of from about 80.degree. to 200.degree. C. The amines employed in this disclosed process are selected from the group consisting of N-methyl aniline, N-ethylaniline, octadecylaniline, diphenylamine, dibenzylamine, pyrrolidone, morpholine, N,N'-diethyl-4,4'-diaminodiphenylmethane, N,N'-diethyl-2,6-tolylene diamine, N,N'-diamino diphenylether and the like.
U.S. Pat. No. 4,618,706 discloses a process for the preparation of aromatic polyisocyanates containing urea and/or biuret groups, optionally in the form of a mixture of homologues and/or isomers by the reaction of aromatic diisocyanate with diamines having primary or secondary amino groups at about 20.degree. to 180.degree. C.
U.S. Pat. No. 3,666,726 discloses polymers containing polyamide, urea and/or urethane groups which are produced by the polyaddition of aliphatic or aromatic polyisocyanates with polyfunctional aminocrotonic acid esters in the presence of a crosslinking agent. In the disclosed process, polyfunctional aminocrotonic acid esters are reacted with polyisocyanates in a ratio of NCO to active hydrogen groups of greater than 1 and preferably 2. This reaction may be carried out in the presence or absence of solvents. Crosslinking agents useful in the disclosed process include water and polyols which may contain hetero atoms such tertiary nitrogen atoms, aminoalcohols, polyamides or diacarboxylic acid hydrazides.
U.S. Pat. No. 5,098,984 discloses an isocyanate-terminated prepolymer having an isocyanate group content of from about 10 to 26% by weight prepared by (1) reacting a C.sub.1 to C.sub.5 alkyl acetoacetate with a polyol having a molecular weight of from about 500 to about 6000 and a hydroxyl functionality of from 2 to 4 and (2) reacting the resultant product with an organic diisocyanate and/or polyisocyanate.
Quite often, the preparation of prepolymers entails the reaction of polyisocyanates with mono- or polyamines at elevated temperatures. The use of high temperatures is uneconomical and can result in product discoloration, and in some cases the liberation of volatile and toxic by-products via a series of biuret exchange reactions. The resultant prepolymers, particularly those derived from diphenylmethane diisocyanates, are not stable liquids.
By the present invention there is provided a facile process for preparing liquid isocyanates containing urea and/or biuret groups.